Add variables to .env to configure the data- and config directory

That adds an way of having the volumes configured easily.

docs/ocis/deployment/ocis_full.md

@@ -129,6 +129,10 @@ See also [example server setup]({{< ref "preparing_server" >}})
   # The default log can be read by machines.
   # Set this to true to make the log human readable
   # LOG_PRETTY=true
+  # Define the oCIS storage location. Set the pathes for config and data to a local path
+  # Note that this can grow big. Leaving it unset stores data in docker internal volumes.
+  # OCIS_CONFIG_DIR=/your/local/ocis/config
+  # OCIS_DATA_DIR=/your/local/ocis/data
 
   # S3 Storage configuration
   #